1. Field of the Invention
This invention relates to stretch hoods. In one aspect the invention relates to mono- and multilayer stretch hoods while in another aspect, the invention relates to stretch hoods in which each of the layers is rich in ultra low density polyethylene (ULDPE).
2. Description of the Related Art
Polyolefin film formulations used in industrial stretch hood packaging applications require a suitable compromise between elastic recovery during packaging and load stability during storage and transportation. Elastic recovery in terms of resin design correlates to low or lower crystallinity resins, while load stability correlates to high or higher crystallinity resins.
Industrial stretch hood packaging film structures have been in commercial use for many years, and typically they are based on ethylene vinyl acetate (EVA) rich film formulations, i.e., formulations of which more than 50 weight percent (wt %) is EVA. The typical film design today is a 3-layer co-extruded structure based on an EVA core layer (most commonly comprising an EVA with a content of units derived from vinyl acetate (VA) of 7.5 to 18 wt %), and skin layers based on metallocene-catalyzed linear low density polyethylene (mLLDPE) rich blends. This type of film structure performs well on packaging lines but at the expense of storage and transportation performance. Specifically, the residual elasticity of such films after the stretching and application process remains high and thus the load stability performance is limited, and EVA resins are weak on-pallet film protection properties such as puncture, tear propagation and dart drop impact performance.
WO 2009/109367 teaches coextruded film structures made using 10 to 95 wt % high melt index ratio (MIR), linear low density polyethylene (LLDPE) in the core layer of a multilayer, blown-extrusion film structure to provide easy processing and strong, highly transparent films. The LLDPE is characterized as having, among other properties, a density of 0.91 to 0.94 g/cm3, a melt index (I2) of 0.05 to 1 grams per 10 minutes (g/10 min), and a MIR (I21/I2) of more than 35. The skin layers of the multilayer film structure comprise at least 85 wt % linear polyethylene (PE) of which at least 75 wt % is an LLDPE with a melt index (MI) of less than 35 and no less than 15 wt % is a high pressure polyethylene (HPPE).
US 2005/0037219 teaches a blown film coextruded tube for use as a stretch hood film in which the core layer of the film comprises EVA or a polymer comprising units derived from ethylene, the polymer having (1) an MI of 0.05 to 20 g/10 min, (2) at least 10 per 1,000 C-atoms of short chain branches containing 5 carbon atoms or less, (3) a density of 0.90 to 0.94 g/cm3, and (4) a relaxation time of at least 10 seconds. The skin layers comprise at least 60 wt % of LLDPE with a density of 0.91 to 0.94 g/cm3 and hexane extractables of less than 1.5 wt %.
EP 1 332 868 B1 teaches a multilayer film which can be used in stretch hood applications, the film comprising a core layer (B) which comprises either (1) 10-90 wt % LLDPE with a density of less than 0.920 kg/m3 and 90-10 wt % of a propylene copolymer comprising 0.3-18 wt % of units derived from ethylene and/or at least on C4-C12 α-olefin and 82-97.7 wt % units derived from propylene, or (2) 60-90 wt % of LLDPE with a density of less than 0.920 kg/m3 and 40-10 wt % of an EVA comprising 5-35 wt % units derived from VA and 65-95 wt % units derived from ethylene. One skin layer (A) comprises low density polyethylene (LDPE) with a density of 910 to 940 kg/m3, a melting point of 106 to 125° C., and a melt flow ratio (190° C./2.16 kg, MFR) of 0.1 to 1 g/10 min. The other skin layer (C) comprises a composition of one or more of propylene/ethylene, propylene/butene and propylene/ethylene/α-olefin copolymers in which the composition has an MFR of 0.5 to 10 g/10 min and a melting temperature of 128 to 145° C.
WO 00/37543 teaches a three-layer film using a blend of a low density, metallocene-produced ethylene/α-olefin copolymer and a predominant amount of EVA with a high amount of VA content in the core layer, and a blend of an ethylene/α-olefin copolymer and a predominant amount of EVA with a low amount of VA content in the skin layers. The skin layers also comprise silicone dioxide as an anti-block friction modifier.
Of continuing interest is industrial stretch hood packaging film that exhibits both packaging and storage and transportation performance.